Conventionally, an occupant detection system for vehicle is known in the art. One system includes a mat shaped electrostatic sensor and an electronic control unit (ECU). The electrostatic sensor may be also referred to as a capacitive sensor. The electrostatic sensor is responsive to a turbulence of a weak electric field which is generated between a main electrode arranged inside a seat and a body member of the vehicle, and is arranged to output the turbulence in a current form or a voltage form. One example is disclosed in JP H11-271463A.
For example, if a seat is vacancy, only air is supplied between the electrodes of the electrostatic sensor. In another example, if a child restraint system (CRS), such as a junior restraint device, a child seat etc. is placed on the seat, CRS or CRS with a child is interposed between the electrodes. Further, if an adult human is on the seat, a large human body is interposed between the electrodes.
Here, the relative dielectric constant of air is about 1.0. The relative dielectric constant of CRS is in a range from about 2.0 to about 5.0. The value varies depending on material of CRS. The relative dielectric constant of a human body is about 50.0. Thus, the relative dielectric constants of air, CRS, and a human body differ, respectively. Therefore, the electrostatic capacity between the electrodes of the electrostatic sensor is changed according to the interposed objects.
The electrostatic sensor is responsive to a turbulence of a weak electric field caused by the difference of the electrostatic capacity. The electrostatic sensor outputs the turbulence in a current form or a voltage form. Then, the ECU performs an occupant determination based on the output. That is, the ECU determines whether the seat is vacancy, the seat is occupied with CRS, or the seat is occupied with an adult. The ECU performs a determination between a disablement and an enablement of activation of the occupant protection device. For example, the ECU determines whether an airbag should be disabled, prohibited, to deploy even if a certain level of collision is detected, or the airbag should be enabled, permitted, to deploy in response to a collision.
In detail, the ECU determines an OFF state where the airbag is disabled, when the seat is vacancy or the seat is occupied with CRS. On the other hand, the ECU determines an ON state where the airbag is enabled, when an adult sits down on the seat. The determination is based on a comparison between a detected value of the capacitance and a threshold value for discriminating the ON state and the OFF state. For example, the ON state is determined when the detected capacitance is equal to or less than the threshold value. The OFF state is determined when the detected capacitance exceeds the threshold value. The ECU may turn on an airbag ON indicator at the ON state, and turns on an airbag OFF indicator at the OFF state.